This invention relates to a process of making modified epoxy resins utilizing a polyepoxide and a mercapto compound. More particularly, this invention relates to a process of making modified epoxy resins via a one-step process.
Epoxy-containing coating compositions are extensively used. Such coating compositions typically contain an organic solvent having the epoxy resin dissolved therein. However, because of concerns as to potential air pollution caused by the solvents in the compositions, a considerable effort has been expended in producing aqueous epoxy-containing coating compositions. Such compositions are either organic solvent free or have had the level of such solvents substantially reduced. Prior attempts to develop water-soluble or water-dispersible epoxy resins have involved reacting the epoxy resin with hydroxy carboxylic acids. Unfortunately, the resultant products have normally been a mixture of different compounds not possessing suitable properties.
U.S. Pat. No. 4,029,621, Hartman et al, issued June 14, 1977 discloses a process of making modified epoxy resins. The described process involves reacting an epoxy resin with a compound containing a mercaptan group and at least one group hydrolyzable to a carboxyl group. The disclosed process includes an hydrolysis step to provide resins with the desired carboxyl functionality which is used for subsequent solubilization. While the resultant products of the U.S. Pat. No. 4,029,621 do possess desirable properties, the described process of making them requires multiple steps and is, for this reason, somewhat cumbersome.
It has now been found modified epoxy resins of the type described in U.S. Pat. No. 4,029,621 can be produced without the necessity of a hydrolysis step. The reaction of a polyepoxide with a mercaptan compound of the formula HS--R--(COOX).sub.m wherein R is an organic radical and X is hydrogen or a moiety formed by reacting a carboxylic group with a basic compound and m is an integer of from 1 to 5 in the presence of a tin catalyst leads to the desired resins without the need of an hydrolysis step.
As used herein all percentages and ratios are by weight unless otherwise indicated.